The transition of the embryonic heart from a single to a dual circulation requires complex remodeling of the cardiac outflow tract (OFT). The remodeling involves shortening and rotation of the myocardial portion of the OFT, and division of the single lumen by septation. These processes result in a mature heart in which the pulmonary artery (PA) connects to the right ventricle (RV) via a myocardial infundibulum in a position anterior and to the right of the aorta (Ao), which connects directly to the left ventricle (LV). In a number of congenital human heart defects, grouped as conotruncal defects, the myocardial infundibulum and/or its alignment with the great vessels is abnormal. It is thus critical to understand the mechanisms by which the mature ventriculo-arterial connections are established. Our initial observation was the co-incidence of OFT cardiomyocyte (CM) apoptosis with the shortening and rotation of the OFT in the transition to a dual circulation. In the previous funding period we used recombinant adenovirus to target inhibitors and activators of the programmed cell death pathway to chick OFT CMs at specific stages of development. Through these apoptosis gain-and-loss of function experiments we showed that perturbations in the extent or timing of apoptosis cause myocardial infundibular defects and anomalous connections of the great vessels with the ventricles that model congenital human conotruncal heart defects. These experiments established the role of CM apoptosis in the remodeling of the OFT in the transition to a dual circulation. Based on data generated in the previous funding period we now propose to test the hypothesis that tissue hypoxia/ HIF-1-dependent signaling induces pro- and anti-apoptotic programs of gene expression that determine the death and survival of the OFT cardiomyocytes during OFT remodeling. In Aim 1 we will use our unique adenoviral gene delivery system to study the effects of HIF-1 gain-and-loss of function. In Aims 2 and 3 we will identify the pro- and anti-apoptotic molecular targets of hypoxia/HIF-1 in the OFT, and then use similar gain-and loss of function experiments to test their function. In each aim, the effect of the perturbation on OFT cardiomyocyte apoptosis, OFT shortening and rotation, and the structure of the ventriculo-arterial connections will be evaluated by biochemical, histochemical and morphological analyses. These studies are aimed at providing a detailed picture of the molecular mechanisms by which PCD is executed in the developing cardiac outflow tract. These studies should also suggest molecules and processes induced by tissue hypoxia that may produce a developmental window of vulnerability to environmental stressors or genetic defects associated with congenital conotruncal defects in the human population.